Familiar designs of transmissions in electric vehicles (EV) are based on conventional transmissions for purposes of coupling to internal combustion engines. These transmissions have an axial offset between the input shaft and the output shaft, in other words, between the rotor shaft of the electric motor (e-motor) and the wheel shafts. The flange shafts leading to the wheels run essentially axis-parallel to the drive machines (electric motor or internal combustion engine) and past them. The drive machine is usually uncoupled in the classic manner, namely, by means of a friction clutch situated directly behind the electric motor. This configuration of the uncoupling mechanism is described, for example, in German patent application DE 10 2007 043 016 A1.
For many electric vehicle applications or hybrid drives, it is sufficient to connect the electric motor to the wheels by means of only one fixed gear transmission or to operatively connect it to the wheels. The electric motor provides a high torque starting right from when the (motor) vehicle is at a standstill, thus dispensing with the start-up element that is necessary in conventional powertrains (with internal combustion engines). The transmission converts the torque of the electric motor, so that the vehicle can start up at its limit of grip. In some vehicle applications, however, the gear transmission ratio defined by this is not sufficient to actually reach the desired final vehicle speed at the maximum rotational speed of the electric motor. In these cases, an additional gear transmission ratio has to ensure that, by means of the electric drive, in other words, by means of the electric motor, not only the required “start-up performance” but also the ultimately desired final speed of the motor vehicle can be achieved. Up until now, however, transmission manufacturers have only produced “two-speed drives” for purely electric vehicles in the axis-parallel design described above.
Thus, gear trains that are structured coaxially to the electric motor are also known that have a transmission section with two planet stages (planetary gears). Here, the transmission section has separate planetary gears coaxially arranged in series, whereby the gear train thus created only requires a small radial and axial installation space and it can provide the requisite total transmission ratio without any problems. Currently, several embodiments of this gear train are being developed and refined; thus, for instance, a coupling mechanism is known which uncouples the electric motor from the powertrain of such a transmission, whereby the uncoupling mechanism is actuated electromechanically and can be integrated into the transmission so as to be essentially “installation-space neutral”.
However, other hybrid drive systems for motor vehicles are also known from the state of the art. For instance, German patent specification DE 44 31 929 C1 discloses a transmission that improves the start-up capacity of a motor vehicle in that there is a separate reduction gear, for example, a planetary gear, located on the intermediate shaft between the drive pinion and another toothed wheel.
European patent specification EP 0 536 230 B1 also discloses a transmission for a motor vehicle which is partially configured as a planetary gear, whereby coupling mechanisms, especially sliding muff-type coupling mechanisms, can serve to couple the internal ring gear of the planetary gear to various components.
German patent application DE 32 30 121 A1 also discloses a hybrid drive system for vehicles with which, in order to achieve a particularly efficient and effective function of such a drive system, the crankshaft (which is configured without a flywheel) of the reciprocating piston internal combustion engine is or can be connected via a first coupling mechanism directly to the rotor of the electric motor that functions as a flyweight for the drive motor and that is or can be connected to the input shaft of the gear train via another coupling mechanism. The construction resources required here, however, are substantial.
Finally, German patent application DE 199 03 936 A1 discloses a transmission for motor vehicles comprising two planetary gears that are each coupled to a transmission shaft on which there are input toothed wheels for various gear transmission ratios. The planetary gears arranged here are situated in the same plane, whereby each planetary gear is to be coupled to a separate electric machine. Here, too, the construction resources required are substantial, especially when two electric motors are to be provided.
Finally, German patent application DE 100 01 602 A1 discloses a transmission in which a planet set or a planetary gear system is employed in order to double the number of speeds that are to be provided, and German patent application DE 197 23 776 A1 discloses an electric single-wheel drive for a vehicle which is intended to achieve a large speed range for the vehicle, whereby a transmission that can shift between at least two rotational speed transmission stages is arranged between the electric motor and the final drive transmission.
A problematic aspect of the hybrid drives known from the state of the art or of the transmissions employed for them is that the construction resources required and the requisite installation space are substantial. This is especially the case when, in order to cover a large speed range for the motor vehicle, the transmission is supposed to be configured so as to be “two-speed”, in other words, to have at least two gears or two gear stages with the appropriate transmission. The shifting mechanisms provided for this purpose and their arrangement or their components require a great deal of installation space in the prior-art transmission systems.